GATE CSE 1992 | Question: 11a

Question

Write syntax directed definitions (semantic rules) for the following grammar to add the type of each identifier to its entry in the symbol table during semantic analysis. Rewriting the grammar is not permitted and semantic rules are to be added to the ends of productions only.

• $D \rightarrow TL;$
• $T \rightarrow \text{int}$
• $T \rightarrow \text{real}$
• $L \rightarrow L,id$
• $L \rightarrow id$

Comments

Similar question: GATE2019-36

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Ref - https://cse.iitkgp.ac.in/~bivasm/notes/SDD.pdf#page=13 page 13

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First understand the language of grammar. Then make a parse tree for some small string. Then try to solve this type of question.

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All questions related to Syntax Directed Definitions(SDD):

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Answer 1

$$\begin{array}{|l|l|} \hline \textbf{PRODUCTION RULE} & \textbf{SEMANTIC ACTIONS} \\\hline  D \rightarrow TL  & L.in:=T.type  \\\hline  T  \rightarrow int & T.type:=integer \\\hline T \rightarrow  real & T.type:=real \\\hline  L \rightarrow L,id & L1.in=L.in \\&Enter\_type(id.entry, L.in)  \\\hline L  \rightarrow  id & Enter\_type(id.entry, L.in)  \\\hline \end{array}$$

Comments

When we are going to perform this action "Enter_type(id.entry, L.in" for the production L$\rightarrow$ L, id?

I mean while inheriting the "in" from Parent or while we perform the reduction.

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Same question (as it is grammar) asked in GATE 2019.

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semantic rules are to be added to the ends of productions only.

This line in the question indicates that this is S-attributed grammar .

and as every S-attributed grammar is L-attributed also (vice versa may not be true).

Thus , it follows all the rules of inherited attributes with constraint( the values can only be inherited from parent node or left sibling).

keeping these points in mind we can write the above semantic rules for the given grammar.

Answer 2

Section 5.2.5 : Semantic Rules with Controlled Side Effects

Textbook : Principles and Techniques of Compiler Design - Ullman